1. Field of the Invention
The present invention relates to a dielectric ceramic used in a microwave region.
2. Description of the Prior Art
Recently, as communication using electromagnetic waves in the microwave region advances, such as mobile telephones, portable telephones and satellite broadcasting, it is desired to downsize the equipment. For this purpose, it is necessary to reduce the size of the individual components composing the equipment. The dielectric ceramics are incorporated in such devices as a dielectric resonator in the filter element or oscillator. The size of the dielectric resonator is inversely proportional to the square root of the dielectric constant of the dielectric material itself when using the same resonance mode, and therefore manufacture of small-sized dielectric resonator requires a material of high dielectric constant. Besides, other requirements of the dielectric ceramics include low loss in the microwave region or the high unloaded Q value, small changes of the resonant frequency with temperature, or small changes of the dielectric constant depending on temperature.
Dielectric materials hitherto used in this field include, among others, Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3, a BaO-TiO.sub.2 composition in the system and their derivatives having some of the elements replaced by other elements. In all these materials, the relative dielectric constant is about 30, and it is too low to downsize the resonator. As a material with a higher relative dielectric constant, BaO-TiO.sub.2 -Sm.sub.2 O.sub.3 composition was disclosed in U.S. Pat. No. 4,330,631 (May 18, 1982). This material has a relative dielectric constant of about 80, a high unloaded Q value of about 3000 at 2 to 4 GHz, and a small temperature coefficient of the relative dielectric constant. In addition, BaO-PbO-TiO.sub.2 -Nd.sub.2 O.sub.3 is known to have a relative dielectric constant of over 90, as reported in the Journal of American Ceramic Society, vol. 67 (1984), pp.278-281.
However, for further reduction of the resonator size, materials of much higher dielectric constant are demanded, but materials possessing both high unloaded Q value and small temperature coefficient are not yet known. On the other hand, the temperature coefficient of the dielectric constant is generally negative when the dielectric constant is higher, but some materials have a positive temperature coefficient value such as PbZrO.sub.3. It is hence attempted to reduce the temperature coefficient by combining materials having positive and negative temperature coefficient values. Materials having negative temperature coefficient include TiO.sub.2 and SrTiO.sub.2, among others, and materials having positive temperature coefficient are represented, for example, by PbO-ZrO.sub.2 -Tb.sub.4 O.sub.7 which is disclosed in U.S. Pat. No. 4,849,384 (Jul. 18, 1989), as the system having lanthanide oxide added to PbO-ZrO.sub.2. This material has a relative dielectric constant of over 100, and contributes greatly to downsizing of the resonator, but the temperature coefficient of the resonant frequency .tau..sub.f is about -1,000 ppm/.degree.C., which shows a large change due to temperature.